A commercially important route to 1,4-butanediol and 2-methyl-1,3-propanediol involves isomerization of propylene oxide to allyl alcohol, followed by hydroformylation of allyl alcohol using a rhodium catalyst to give a mixture of 4-hydroxybutanal and 3-hydroxy-2-methylpropanal. Catalytic hydrogenation of the aldehyde mixture gives mostly 1,4-butanediol and a minor proportion of 2-methyl-1,3-propanediol. The diols are useful chemical intermediates for making polyesters, polyurethanes, thermoplastic elastomers, and other useful products.
The aldehyde mixture from hydroformylation is usually separated from the rhodium catalyst solution by extraction into water. This aqueous stream is a relatively inexpensive source of both 4-hydroxybutanal and 3-hydroxy-2-methylpropanal. Each of these compounds is valuable in pure form as a chemical intermediate for making pure diols useful in polyesters, polyurethanes, and alkyd resins. Unfortunately, the similarity in boiling points and solubility characteristics of the aldehyde isomers complicates their separation and use as pure compounds.
Matsumoto teaches that 4-hydroxybutanal can be separated from aqueous media by vacuum distillation (see Japanese Patent Application Kokai No. 3-261775), but does not teach how to separate 4-hydroxybutanal from 3-hydroxy-2-methylpropanal.
In copending application Ser. No. 07/868,050, filed Apr. 13, 1992, now U.S. Pat. No. 5,254,702 we disclose a two-phase process for producing a 2-oxytetrahydrofuran by reacting an aqueous solution of 4-hydroxybutanal with a hydroxy compound in an organic solvent in the presence of an acid catalyst. The 2-oxytetrahydrofuran product is isolated from the organic phase by any suitable method. Isolation of individual aldehyde isomers from the aqueous aldehyde mixture is not taught.
A practical process for isolating 4-hydroxybutanal and 3-hydroxy-2-methylpropanal in pure form is needed. In particular, a process that overcomes the need to separate the isomers based on boiling point or solubility differences would be valuable. Preferably, the process would be effective to cleanly and efficiently isolate either or both aldehyde isomers.